Western Blotting

Western blot analysis of extracts from T-47D cells, grown for 48 hr in phenol red-free medium supplemented with 5% charcoal-stripped FBS and then treated with either a vehicle control (-) or promegestone (R5020, 100 nM, 16 hr; +), using Progesterone Receptor A/B (D8Q2J) XP® Rabbit mAb (upper) or GAPDH (D16H11) XP® Rabbit mAb #5174 (lower). Prolonged treatment of PR-expressing cells with R5020 is known to induce PR downregulation and hyperphosphorylation, which is reflected by slower migration on SDS-PAGE.

Nuclear Receptors are transcription factors responsible for sensing bioactive molecules, including steroid and thyroid hormones. They are regulated by multiple posttranslational modifications, which in turn impacts their ability to regulate the expression of specific genes involved in the control of reproduction, development, and metabolism.

Androgen receptor (AR), a zinc finger transcription factor belonging to the nuclear receptor superfamily, is activated by phosphorylation and dimerization upon ligand binding (1). This promotes nuclear localization and binding of AR to androgen response elements in androgen target genes. AR plays a crucial role in several stages of male development and the progression of prostate cancer (2,3).

Glucocorticoid hormones control cellular proliferation, inflammation, and metabolism through their association with the glucocorticoid receptor (GR)/NR3C1, a member of the nuclear hormone receptor superfamily of transcription factors (6).

Peroxisome proliferator-activated receptor γ (PPARγ) is a member of the ligand-activated nuclear receptor superfamily and functions as a transcriptional activator (7). PPARγ is preferentially expressed in adipocytes, as well as in vascular smooth muscle cells and macrophages (8). Besides its role in mediating adipogenesis and lipid metabolism (8), PPARγ also modulates insulin sensitivity, cell proliferation, and inflammation (9).

Human progesterone receptor (PR) is expressed as two forms: the full length PR B and the short form PR A. PR A lacks the first 164 amino acid residues of PR B (10,11). Both PR A and PR B are ligand activated, but differ in their relative ability to activate target gene transcription (12,13).

Nuclear retinoic acid receptors (RARs) consist of three subtypes encoded by separate genes: α (NR1B1), β (NR1B2), and γ (NR1B3). For each subtype, there are at least two isoforms, which are generated by differential promoter usage and alternative splicing and differ only in their N-terminal regions. Retinoids, which are metabolites of vitamin A, serve as ligands for RARs (14). RARs function as ligand-dependent transcriptional regulators and are found to be heterodimerized with retinoid X receptors (RXRs). These transcriptionally active dimers regulate the expression of genes involved in cellular differentiation, proliferation, and apoptosis (15,16).